Cryosurgery is a well established surgical method which is useful in the treatment of many conditions and which involves the application of extreme cold to tissues to effect freezing of the tissues. Cooling and defrosting in such a method is achieved by a variety of methods. Considering only probe-type instruments, as opposed to direct topical application of a cryogen, cryosurgery may include the introduction of a low boiling point refrigerant into a closed probe tip, gas expansion utilizing the Joule-Thompson effect, employing the latent heat of vaporization such as with freon, precooled gases and liquids, or thermoelectric cooling.
A preferred form of cryosurgery employs a closed end probe through which a low boiling point refrigerant or a Joule-Thompson expansion fluid is circulated. Such a closed end probe confines the cryogenic fluid within the instrument rather than applying it directly to the tissues being treated. In closed end probes, the cold generated by a cryogenic fluid is confined to the area of the probe tip where heat transfer occurs across the probe tip to the surrounding tissues and forms an ice ball therein.
Prior art probes have generally been manufactured by methods which introduce potential points of weakness or reduce the effectiveness of the devices. For example, common prior methods of construction include press-fitting of parts, soldering and welding, e.g., Tig welding. Press-fitting as in, for example, U.S. Pat. No. 3,439,680 to Thomas, Jr., is inaccurate and prone to leakage which results in wastage of cryogen and potential damage to surrounding healthy tissues. Soldering and welding are labor intensive and require a degree of precision which renders mass production of probes significantly expensive. In addition, soldering and welding introduce compounds, such as fluxes and oxides, which are difficult to remove from the final probe assembly, particularly the tight, confined areas within the probe, and, if left, can cause degradation of the joints and leakage of cryogen. Furthermore., in these prior methods of manufacture, if a vacuum is desired in a probe handle to provide thermal insulation a separate and complicated evacuation procedure is necessary. Maintenance of such vacuums is also subject to the same deficiencies in the joints produced by prior methods.
The inventors herein have devised a probe construction and a method of manufacture therefor which overcomes the deficiencies of the prior art. By employing a vacuum brazing technique, they have produced a cryosurgical probe with improved construction and properties which have heretofore been, at best, difficult to obtain. The construction and method herein also render mass production of probes more economical while permitting the probes thus made to have a high quality and consistent uniformity of construction. In addition, the evacuation of the probe handles and the improved maintenance of that vacuum is more easily and readily obtained and is an integral part of the final assembly steps rather than a separate procedure following manufacture.
At the heart of the present method is the use of vacuum brazing to both draw a vacuum simultaneously on the handle portion and a substantial length of the probe shaft for thermal insulation and to join the individual parts of the probe together. Brazing is an adhesion process in which the base metals being joined are heated to a temperature at which a nonferrous filler, or brazing material, melts and is distributed between the closely fitted surfaces of a joint by capillary attraction. Instead of melting the base materials, as in a welding process, brazing joins the base materials by melting only the brazing material and doing so at temperatures at which a degree of alloying may occur between the base metal and brazing material. The resulting joint is smoother and more uniform and is generally less subject to the effects of stress than welded or soldered joints.
In vacuum brazing, the components, with brazing material pre-positioned at the joint locations, are placed in a furnace and subjected to brazing heat under a vacuum. The use of vacuum eliminates the need for mineral fluxes that are used in welding and soldering and which can leave residual contaminants on the surfaces of the probes. The brazements are then cooled or quenched by appropriate protocols to minimize distortion and produce the required properties in the base and brazing materials. Commercial vacuum brazing is generally accomplished at pressures varying from 0.5 Torr to 10.sup.-6 Torr, depending on the applications. Vacuum brazing has typically been used to fabricate vacuum tubes for electronic devices, as well as bodies of similar and dissimilar metals including stainless steel, super alloys, aluminum alloys, refractory materials and ceramics. The following patents are representative of such brazing methods and processes: U.S. Pat. No. 2,800,711, Oliphant, et al.; U.S. Pat. No. 2,822,609, Horvitz; U.S. Pat. No. 2,943,181, Gunow, et al.; U.S. Pat. No. 3,512,245, Hermann; U.S. Pat. No. 4,081,121, Picard; U.S. Pat. No. 4,118,542, Walter; U.S. Pat. No. 4,401,254, Tramontini; U.S. Pat. No. 4,804,128, Brittin, and U.S. Pat. No. 27,733, Bereza, the disclosures of which are incorporated herein by reference thereto.
In the construction of cryoprobes, the inventors have found that vacuum brazing provides advantages over other joining means, including soldering, welding, epoxy, or other brazing methods. Although brazing is a known method and has been disclosed in the patent art as a possible alternative for partial construction of cryogenic probes, (for example, Thomas, Jr., U.S. Pat. No. 3,439,680, discloses brazing a stainless steel tip onto the end of a probe and Ritson, et al., U.S. Pat. No. 3,913,581 discloses brazing a thin walled stainless steel tube into a cylindrical body), the present inventors have found no evidence suggesting the use of vacuum brazing in the manner herein described for the complete construction and evacuation of cryoprobes. The present invention has the following advantages over probes manufactured according to prior methods.
As noted, the vacuum brazed cryoprobe of the present invention eliminates the need for separate pumping to evacuate the handle as well as the probe shaft beyond the freezing zone for thermal insulation. A vacuum is drawn on the probe by the vacuum level of the brazing furnace and the handle is then sealed by the continued vacuum brazing. A sufficiently high vacuum level in the probe's vacuum chamber can be obtained by evacuation through loosely pre-positioned brazing joints or through an evacuation valve, e.g. a valve which is activated and itself sealed simultaneously with the brazing. An evacuation valve is preferred and may be necessary when sufficient evacuation paths are not available, such as when each probe component is tightly fitted during the pre-assembly process as described below. Since the vacuum is produced by the furnace which has a high vacuum applied (at most 10.sup.-3 Torr and preferably at most 10.sup.-5 Torr or lower in pressure, i.e. higher vacuum) as well as the high brazing temperature, and since sealing occurs substantially simultaneously, the evacuation is more complete and can provide better thermal insulation than that obtained by other methods. Even higher vacuums may be achieved by incorporation of a getter in the probe body.
Vacuum brazing is a fluxless process and creates more uniform and cleaner joints than either soldering or welding. Because it is fluxless, there are no residual contaminants of flux or oxides to be cleaned out. Such contaminants may present problems in fine probe production since they can affect the quality of the joints as well as potentially degrade the joints thus affecting the vacuum insulation. In addition, the elimination of fluxes and their oxides reduces the oxidation of the probe material and significantly reduces the level of outgassing by the probe material. This reduction of outgassing improves the vacuum level which is attained in the probes, it reduces the level of joint contamination and it increases the shelf life of the probes. Vacuum brazing removes essentially all occluded gases evolved at close fitting brazing interfaces and, because the probe components have been "pre-outgassed" at the high furnace temperatures and vacuum, any outgassing subsequent to probe construction is minimal thus substantially extending the shelf life of the probes of this invention.
The method of this invention permits high volumes of production with consistent and uniform quality since a plurality of probes may be produced with each run of the furnace. The number of probes produced is primarily limited by the capacity of the furnace used. Compared to welding and soldering, vacuum brazing is more efficient and human error is minimized. Every heating and cooling stage is controllable by automatic means ensuring continuity throughout the process. Furthermore, the high volume and economy of production obtained by vacuum brazing justifies the disposability of the probes produced thus reducing the problems of defects which may arise in re-use of probes manufactured by other methods.
In addition, certain properties, such as probe stiffness or flexibility, which are imparted by the vacuum brazing process are more readily altered or adjusted to need simply by changing the process steps. Although flexible cryosurgical probes have been made before, the flexibility of the probe is generally obtained through a complex construction employing materials which may deteriorate under extreme cold and in which the degree of flexibility is not readily altered without a change in the structure of the devices. Such probes are represented by U.S. Pat. No. 5,078,713, Varney; U.S. Pat. No. 5,108,390, Potocky, et al. and U.S. Pat. No. 5,139,496, Hed. Other probes employ materials which, while having the flexibility to be shaped and the stiffness to retain such shapes, are potentially toxic. For example, U.S. Pat. No. 4,072,152 to Linehan discloses an orthopedic cryosurgical apparatus employing probes formed of lead which are placed within the body. Lead is chosen for its flexibility. However, the annealing effect of the vacuum brazing process of the present invention has the advantage of providing a method whereby the flexibility or stiffness of more common construction materials, such as stainless steel, can be tailored to the point where they can be easily bent or shaped while retaining the desired flow characteristics of the cryogenic fluid. Furthermore, the process allows probes to be pre-shaped then set to retain that shape.
In addition, vacuum brazing produces probes which are safer to use in a clinical environment since there are fewer joints, fewer contaminants and fewer materials. Furthermore, the alloying and annealing which can take place in vacuum brazing produces hermetic joints which are stress free and can better withstand the drastic thermal shocks encountered in cryosurgery. The joints also have a higher ductility and, since they may be substantially all internal rather than external as with welded joints, there is less exposure of joint areas to patient contact.
While the application of the vacuum brazing process as described herein is broadly applicable to any cryosurgical probe construction, particularly those relying on vacuum insulation, the structure of the preferred embodiments of the probe of this invention also presents advantages over the prior art. Both the structure and the method of manufacture make it easier to effect changes in the freezing chamber during manufacture to produce probes which can generate different sizes and shapes of ice balls. Furthermore, the construction of the preferred probe presents a new design which is slimmer and which permits easier positioning of multiple probes in a limited surgical area. Also, the vacuum brazing process produces probes which are extremely clean internally and externally and which do not suffer from discoloration produced by welding or soldering.
The vacuum brazed cryoprobes of this invention may be advantageously used in the assignee's cryosurgical system employing sub-cooled liquid nitrogen as disclosed in copending application Ser. No. 07/953,279 and U.S. Pat. No. 5,334,181, the disclosures of which are incorporated herein in their entirety by reference thereto. Moreover, the invention vacuum brazed cryoprobes also most advantageously and preferably include a vented cryogen supply tube as described in assignee's recently issued U.S. Pat. No. 5,254,116 and in assignee's copending application Ser. No. 08/137,353, the disclosures of which are incorporated herein in their entireties by reference thereto.
It has been observed, however, in studies carried out by and on behalf of one or more of the present inventors that under some conditions of use and for some parameters the operating performance of the invention vacuum brazed cryoprobes was inferior to that of the assignee's current commercially available 3 millimeter (mm) Tig-welded cryoprobes, which are sold as part of the assignee's AccuProbe.RTM. cryosurgical system. For example, the external temperature near the probe tip often ranged from about 20.degree. C. to about 50.degree. C. higher for the vacuum brazed cryoprobe as compared to the Tig-welded cryoprobe. One probable cause for this lowered performance was determined to be the location of the opening of the vacuum valve 41 as shown in FIG. 1 forward of the enlarged return chamber, resulting in an uninsulated, or relatively poorly insulated return chamber. While the lower external surface temperatures are still well below 0.degree. C. and capable of freezing and destroying tissue, it is preferred to operate at as low a temperature as possible, consistent with adequate safety and other economic considerations, to minimize the time required for the surgical procedure.
Accordingly, in a particular aspect of the invention, specific design changes are incorporated in the preferred cryoprobe instrument which have resulted in the desired external temperatures at and near the probe tip as well as minimizing cool down time, liquid nitrogen consumption and other operating parameters. These design changes include, in particular, providing a nozzle at the outlet of the cryogen supply tube, and/or optimizing the vent hole pattern in the cryogen supply tube. In addition, the improved vacuum insulation may be extended to the proximal (rear) end of the handle portion corresponding to the enlarged return chamber.
While one or more of these design modifications may be incorporated into the cryoprobe to achieve maximum operating performance, the overall benefits, as discussed previously, for a vacuum brazed cryoprobe, are not dependent on the incorporation of these additional design features.